1. Field of the Invention
This invention generally relates to the field of electro-optic readers such as laser scanners for reading bar code symbols and, more particularly, to an arrangement for, and a method of, enhancing the depth of modulation of an analog electrical signal indicative of each symbol being read under circumstances where the modulation has been degraded.
2. Description of Related Art
Various optical readers and optical scanning systems have been developed heretofore to optically read bar code symbols printed on labels affixed to objects in order to identify the object by optically reading the symbol thereon. The bar code symbol itself is a coded pattern of spatially adjacent elements comprised of a series of bars (dark elements) of various widths, and spaced apart from one another to bound spaces (light elements) of various widths, the bar and space elements having different light-reflecting characteristics. The various element widths were first converted to an electrical analog signal. The analog signal was then digitized to an electrical digital signal. The digital signal was then electronically decoded to a multiple alpha-numerical digit representation descriptive of the symbol and, hence, of the object. Scanning systems of this general type, and components for use in such systems, have been disclosed, for example, in U.S. Pat. Nos. 4,251,798; 4,360,798; 4,369,361; 4,387,297; 4,593,186; 4,496,831; 4,409,470; 4,460,120; 4,607,156; 4,673,805; 4,736,095; 4,758,717; 4,760,248; 4,806,742 and 4,808,804; as well as in U.S. patent application Ser. Nos. 196,021; 944,848; 138,563; 148,669; 148,555; 147,708; 193,265; 265,143; 265,548; 265,149 and 264,693; all of which have been assigned to the same assignee as the instant application and are incorporated herein to show the state of the art.
As disclosed in some of the above patents and applicatons, a particularly advantageous embodiment of such a scanning system resided, inter alia, in optically modifying and directing a laser light beam from a hand-held head which was supported by a user; aiming the head and, in some cases, the laser beam itself at a symbol to be read; repetitively scanning the laser beam and/or the field of view of a detector supported on the head in a scan direction across the symbol; detecting the laser light reflected off the symbol during scanning, the reflected light having a variable intensity over the scan since the bar elements reflect less light than the space elements; converting the reflected laser light of variable intensity into an electrical analog signal; digitizing the analog signal into an electrical digital signal; and decoding the digital signal.
The laser beam is optically modified and focused by an optical train, which typically includes a focusing lens, to form a beam spot having a minimum beam cross-section or waist at a reference plane. The size of the beam spot increases on either side of the reference plane, the range of distance wherein the symbol can still be read by the beam spot being known as the "depth of focus". The dimension of the beam spot along the scan direction may or may not be equal to the dimension of the beam spot along a direction perpendicular to the scan direction. A symbol can be read at either side of the reference plane. For ease of description, a symbol located between the reference plane and the head is defined as a "close-in" symbol, whereas a symbol that is located on the other side of the reference plane away from the head is defined as a "far-out" symbol. The term "close-in" symbol is also intended to cover the situation where the symbol actually is contacted by the head, or where the reference plane is located immediately outside the head. The range between minimum and maximum distances at which the system can read a symbol is often defined as the "depth of field". The depth of field is, of course, different for symbols of different densities. A high density symbol is characterized by very thin bar elements spaced very closely together so that the symbol occupies very little physical space, whereas a low density symbol is characterized by very wide bar elements spaced very far apart so that the symbol occupies a much greater physical space.
A problem associated with known laser-based and non-laser-based scanning systems, as well as wands that are manually swept across each symbol, and charge-coupled device (CCD) scanners relates to the loss of information about the symbol being scanned. Under certain circumstances, the loss of information is so great that the symbol cannot be successfully decoded and read.
The capability of the system to read symbols is determined at least in part by the size of the sampling aperture in relation to the sizes of the elements of the symbol. In a moving laser beam scanner where the outgoing laser beam itself is directed to, and swept across, the symbol, the sampling aperture is the diameter of the laser beam at the symbol, or, more particularly, the sampling aperture is the dimension of the laser beam in cross-section at the symbol as considered along the scan direction. In scanned aperture readers, the sampling aperture is the field of view of a photodetector that is scanned across the symbol. In retro-reflective readers where both the outgoing laser beam and the field of view of the photodetector are simultaneously scanned across the symbol, the sampling aperture is, once again, the beam diameter.
Traditionally, scanners have been designed with a sampling aperture of about the same size or width as the width of the narrowest bar or space element in the symbol to be scanned as considered along the scan direction. The photodetector that "watches" the sampling aperture during scanning collects light reflected off the symbol and generates an electrical analog signal indicative of the detected reflected light. The reflected light has a variable intensity across the scan, thereby causing the analog signal to constitute a succession of analog pulses whose amplitudes correspond to the amount of reflected light collected by the photodetector. Each analog pulse has a width proportional and ideally equal to the width of each bar element, and an amplitude which depends on the size of the sampling aperture relative to each bar element. By designing the sampling aperture to be about equal to the narrowest bar element, the amplitudes for both narrow and wider bar elements are approximately the same in the analog signal. Put another way, the analog signal has a nearly equal modulation amplitude for both narrow and wider bar elements.
The nearly equal modulation amplitude is very desirable to facilitate the functioning of the analog-to-digital digitizer circuit that is used to convert the analog signal to a digital signal composed of a succession of digital pulses, each having widths proportional to the widths of the corresponding analog pulses and, in turn, to the widths of the bar and space elements in the symbol being scanned.
However, a nearly equal modulation amplitude for the analog signal is not often available in practice. The aforementioned depth of focus is directly proportional to the size of the sampling aperture. It is often desirable to have scanners with a very long depth of focus so that each symbol can be read when positioned anywhere within an extended range of distances relative to the scanner. However, a long depth of focus dictates a correspondingly large size for the sampling aperture which, in turn, causes the modulation amplitude to degrade because the large sampling aperture will overlap the narrowest bar element or space element.
A larger sampling aperture is also desirable in practice in order to provide a better signal-to-noise ratio for the analog signal because more light reflected from the symbol can be collected through a larger sampling aperture.
Modulation can also be degraded by electrical low pass filtering typically employed to reduce the level of electrical noise in the system.
Analog-to-digital digitizer circuits can "square up" a modulation-degraded analog signal to some extent so that the symbol can be successfully read. All digitizer circuits, however, will eventually fail when the modulation becomes too poor.